The present invention relates to the field of electromechanical microactuators, that is to say the field of microsystems adapted for delivering a controlled mechanical force in response to an electrical excitation.
The document U.S. Pat. No. 5235225 describes a structure in which the basic actuator consists of two parallel stators each furnished with a plurality of electrodes and with a flexible rotor, which is at least partially electrically conductive and disposed between the two aforesaid stators. The displacement of the rotor is effected through sequential control, progressing in space, from an electric voltage applied to the electrodes.
More precisely still, the present invention relates to the field of electrostatic microactuators known as xe2x80x9cScratch Drive Actuatorsxe2x80x9d or xe2x80x9cSDAsxe2x80x9d. Specifically, the present invention relates to an electrostatic microactuator based on distributed elementary SDAs.
A description of these actuators will be found in the documents [1], [2] and [3].
These actuators proposed some years ago, are most particularly intended for the direct driving of micromachines of micrometer dimensions. They have the particular feature of associating a mechanism for transferring mechanical energy by friction with the conventional implementation of an electrostatic force field.
The aforesaid documents may usefully be referred to for a proper understanding of the general structure and the operation of these actuators.
The latter are shown diagrammatically in FIGS. 1A to 1D attached.
In essence, an SDA comprises a plate or beam (10), made for example of polysilicon, furnished at one end with a projecting strut or pad 12, directed toward a substrate 20, made for example of silicon, covered with an insulating layer 22, made for example of Si3N4. A generator 30 is adapted for applying voltage pulses between the plate 10 and the substrate 20.
As may be seen in FIG. 1B, on a rising pulse edge, the plate 10 is drawn toward the substrate 20 by the electrostatic force generated between the latter. The bearing of the strut 12 on the layer 22 imposes a static flexion on the plate 10, which in turn gives rise to an offset of the strut 12.
On the falling edge of the pulse, as may be seen in FIG. 1C, the plate 10 tends to revert to its rest geometry, by virtue of the elastic energy stored up in the plate 10, and has therefore been shifted by a flexion of the plate 10, which in turn gives rise to an offset of the strut 12.
On the falling edge of the pulse, as may be seen in FIG. 1C, the plate 10 tends to revert to its rest geometry, by virtue of the elastic energy stored up in the plate 10, and has therefore been shifted by an amplitude dx with respect to its former position, by reason of the bearing defined between the strut 12 and the layer 22.
Thus, these systems make it possible to convert mechanical oscillations of very small amplitude originating in the static flexion of the thin plate 10, into a rigid body motion of this same plate.
The lower the height of the strut 12 situated under the flexing plate 10, the stronger the electrostatic forces produced at the plate 10/substrate 20 interface, for a given excitation voltage emanating from the generator 30. The height of the strut 12 typically of the order of a micrometer, moreover introduces a high gearing reduction in the energy conversion mechanism at the plate 10/substrate 20 interface. The gearing reduction intrinsic in the very small mechanical deformations involved in energy conversion by friction, contributes to a dual increase in the driving forces generated during the displacement of the plate. SDAs thus have the particular feature of developing sizeable useful forces at low speed, in the absence of any auxiliary speed decrease.
The length of the displacement step depends on the height of the strut 12, on the stiffness of the plate 10 and on the control voltage applied. The displacement step is typically of the order of 25 nanometers for a plate 10 exhibiting a width of the order of 50 micrometers, a thickness of the order of 1 micrometer and a length of the order of 60 micrometers.
The repeating of such cycles makes it possible to accumulate displacement steps and consequently allows a sizeable relative displacement between the plate 10 and the substrate 20.
However, although they are showing themselves to be very promising, to the knowledge of the inventors, SDAs have hitherto remained on the laboratory scale and have not enjoyed industrial development.
This seems to be due in particular to the fact that the force generated by the known SDAs remains limited even if it is sizeable on the micrometer scale. This force, typically of the order of from 50 to 100 micro newtons for an SDA energized at a peak excitation voltage of the order of 100 V, can satisfy only a limited number of applications reserved exclusively for the micromachine scale.
Also, attempts to appreciably intensify this force by increasing the size of the SDAs have not been crowned with success hitherto.
Indeed, on the one hand the electrostatic forces involved in the actuation decrease very rapidly with increasing dimensions of the SDAs. On the other hand, the production processes involved in fabricating SDAs prevent the production of devices having a thickness greater than a few microns, which constitutes an intrinsic limitation to the increasing of the other dimensions of the SDA.
The object of the present invention is now to propose novel means making it possible to implement SDAs industrially.
Contrary to current attempts tending to increase the size of an SDA so as to obtain an acceptable output force, within the framework of the present invention it is proposed to retain SDAs of small size, but to increase their number and to associate them under suitable conditions so as to allow the addition of the forces generated by each of these SDAs, namely by using means adapted for, on the one hand, applying to said SDAs an external mechanical prestress able to allow a superposition of the forces generated by the various SDAs and, on the other hand, communicating to an external load the entire driving force emanating from the collective behavior of these same SDAs.
This mechanical prestress of the SDAs is advantageously obtained with the aid of a bias voltage applied at rest to the set of SDAs.
To allow the entire driving force emanating from the collective behavior of the SDAs to be communicated to an external load, according to an advantageous characteristic of the present invention, the sheet carrying the SDAs is placed in a mechanical clearance at the interface of two solid bodies articulated together.
The inventors have in fact demonstrated that such a prestress, associated with means guaranteeing the communication of the driving force, to the external load, was indispensable for allowing aggregation of the forces generated by the various SDAs.
The cooperation of microactuators has already been utilized in the field of the motorization of micromachines but, to the knowledge of the inventors, only via the development of conveyors of objects by friction in the horizontal plane, so as to profit from the gravity of the displaced object. With such devices, the driving forces communicated to the movable element depend only on the mass of the displaced object, as well as on the coefficient of friction at the object/actuators interface (in accordance with Coulomb""s laws of solid friction) . In this case the driving forces communicated to the displaced object are unaffected both by the number and by the driving characteristics of the actuators participating in the motorization. Moreover, these same driving forces depend on the configuration of the machine (or conveyor) in space and in particular on the horizontality of the object transfer plane. It is clear, consequently, that increasing the number of actuators participating in the motorization of one and the same load does not necessarily lead to a matching increase in the useful forces involved in the motorization.
The present invention is distinguished from the previous inventions in that it does not use gravity (or any other solution such as the elastic deformation of a bearing spring, etc.) to calibrate a prestress in the mechanical power transmission procedure. It uses electrostatic attraction forces which are particularly strong in view of the scale of the SDAs, to calibrate an individualized prestress for each actuator involved in the collectivity. This prestress is intrinsic to each actuator insofar as it is unaffected by the parameters of the displaced load, unlike the prior art devices which routinely involve gravity. The application of the prestress, within the framework of the present invention, is moreover natural since the electrostatic attraction does not require recourse to the elastic deformation of an auxiliary bearing spring. In practice, this deformation is obtained with the aid of a bias voltage applied at rest to the set of SDAs, as indicated earlier. Moreover, the electrostatic forces depend only on the relative position of the SDA on its substrate and are unaffected by the posture of the substrate in three-dimensional space.
The proposed invention consequently guarantees, unlike the collective devices of the prior art, effective superposition of the individual forces of each SDA, regardless moreover of the spatial configuration of the colony considered.
The inventors have moreover found that the support lattices proposed hitherto, and capable of associating several SDAs, turn out to be incapable of retaining their mechanical integrity during the transmission of sizeable external forces. These known lattices are generally formed of an assembly of extremely fragile beams having, like the SDAS, a thickness of the order of a micrometer (these being fabricated at the same time as the SDAs, in the course of the same procedure and from a similar material). The fragility of the known lattices is therefore an intrinsic limitation to the transmission of sizeable forces.
Thus, according to another advantageous characteristic of the present invention, contrary to current attempts, the inventors propose that flexible sheets, made for example of polysilicon, comprising a large number of SDAS, be inserted into a mechanical clearance separating two solid bodies articulated with respect to one another. This technical solution in fact allows a very large number of SDAs to be made to cooperate in practice under conditions promoting effective superposition of the driving forces and in such a way that the transmission of mechanical power, which results from the accumulating of the useful forces produced by the collectivity, may not be passed on to the hardware structure connecting the assembly of SDAs.
To this end, within the framework of the present invention, the SDA sheets advantageously consist of a framework, in contact solely with one of the solid bodies involved in the articulation, for example the bedplate. The SDAs are for their part only in exclusive contact with the other solid body involved in the articulation, for example the drive shaft. Such a configuration permits accumulation of force proportional to the number of SDAs involved in the interface. It also and above all guarantees the integrity of the framework (since the latter is reinforced on contact with the bedplate), regardless of the transmission of external mechanical power communicated to the movable element (or drive shaft).
The invention thus contributes, as a whole, to the physical and material (or mechanical) possibility of involving a very large number of SDAs in the driving of one and the same load, unlike in the case of the known prior art solutions.
Typically, a system in accordance with the present invention thus integrates from a few tens to a few thousand SDAs.
Moreover, the present invention proposes specific means for shaping a sheet, made for example of polysilicon, comprising a large number of SDAs, by flexion of bars integral with this sheet.
The present invention also relates to a process for shaping a sheet comprising a large number of SDAs, utilizing such means.
According to another advantageous characteristic of the present invention, the system can also comprise means forming a force sensor, for example a torque sensor, comprising at least one beam integrated into the sheet forming the SDAs and adapted to be deformed upon actuation of the system, said beam being associated with means for analyzing its deformation.